This new proposal submitted in response to the NIMH Program Announcement (PA-00-119) for the Behavioral Science Track Award for Rapid Transition (B/START) is consonant with the goals of this funding mechanism. We request one year of funding to develop and evaluate an age-downward extension [into adolescent populations] of an empirically validated treatment for adult and geriatric major depressive disorder (MDD). Adolescent MDD is an important public health problem. The development of effective treatments for this disorder has significant public health implications. Although currently available treatments for adolescent MDD are encouraging, research shows that between 25% to 45% of adolescents fail to respond to state-of-the-art interventions (e.g., cognitive-behavior therapy and selective serotonin reuptake inhibitors). A number of well-controlled studies have shown that supervised aerobic exercise is as effective as antidepressant medication and psychosocial interventions for the treatment of adult MDD. An aerobic exercise intervention has the potential to be a viable alternative or adjunctive treatment to psychosocial and psychopharmacological interventions. However, the therapeutic viability of aerobic exercise for the treatment of adolescent MDD has not be systematically evaluated. The goal of the proposed research project is to develop and evaluate a 12-week supervised aerobic exercise intervention for the treatment of adolescent MDD. A total of sixteen depressed outpatient adolescents (ages 13 to 17, inclusive) will be recruited to participate in one of two pilot studies. The primary purpose of the first pilot study (n = 4) is to develop and test a manualized aerobic intervention and revise it according to subject feedback. The purpose of the second pilot study (n = 12) is to examine the feasibility, acceptability, sufficiency, and efficacy of the manualized aerobic intervention in the acute treatment of adolescent MDD. The pilot studies will also provide an estimated effect size that will be used to power future studies in this area. If empirically supported, the investigators will pursue further treatment development and evaluation in larger clinical trials.